1. Field of the Invention
This invention relates to apparatus for generating charged particle beams, e.g., electron and ion guns. Such beams are typically utilized to bombard surfaces to cause the emission of particles therefrom, which particles may then be analyzed to determine the composition or other characteristics of the surface.
2. Background of the Prior Art
One type of apparatus for generating a charged particle beam, especially for generating a highly focused beam of positive ions, ionizes gas atoms in an electrical discharge and extracts the ions thus formed by suitable electric and/or magnetic fields to form the beam. Such plasmatron sources are capable of providing intense beams; however, the high energies involved in the plasma result in an ion beam containing ions in a number of different ionization states, which have a considerable spread in energy and which contains ions of a variety of elements. Filtering of such a beam is often required if the beam is to be used in applications where a monoenergetic beam is required.
Another general type of ion gun for directly producing a beam having a considerably smaller spread in energies utilizes low voltage electron bombardment of gas atoms such that substantially all of the energy of the ions in the ultimately produced beam is provided upon acceleration. One such gun is disclosed in U.S. Pat. No. 3,665,182 (Goff & Smith). While such a device is capable of providing a substantially monoenergetic beam, it is incapable of providing a highly focused beam. If apertured to a small diameter, the current would be reduced to an undesirable extent for such purposes as forming an image representing surface characteristics.
U.S. Pat. No. 3,840,743 (Tamura & Kondo) discloses an ion gun including two focusing lenses and a plate having a relatively large diameter aperture positioned therebetween. A plasmatron type ion source having a small source diameter is disclosed. The spot diameter of the beam is said to be variable to any desired value from 1 micrometer to several hundred micrometers. Such a variation is readily produced by changing the potential in the first focusing lens such that a variable amount of the beam is blocked by the aperture plate. However, as discussed hereinabove, the energy spread and varying composition of such a beam precludes its use in many applications.